Random choices include :- ??
If you add the following info, someone may be able to offer more advice.
Ram size (256 is early 512 is later version which have some issues with certian distributions)
SD card size
SD card type (ie class4)
Distribution on the card.
What does it do LED wise
Do you get any sort of picture.
Have you measured the voltage ( http://elinux.org/R-Pi_Troubleshooting#Troubleshooting_power_problems )
Did you change anything on the time it booted.
Have you reloaded the distribution onto the/another card.
Otherwise its a random guess.....
I take it that "looks to be yellow" is in response to
"What does it do LED wise".
If _all_ you are seeing is a yellow LED, I would suspect a power supply problem.
After power up, the red PWR led should be lit and you should see activity on the green led to its right as it boots from the SD card.
There is then a short delay before the other leds come on and start showing activity.
If you don't get the red one, something is wrong with the power for sure.
I suggest starting with the RasPi Troubleshooting Wiki.
If you were able to boot once and now you can't, the usual suspects are:
1. Corrupted SD card from an unclean shutdown (always shut down using a GUI command or the GNU/Linux "shutdown" command).
2. Polyfuse F3 is misbehaving and has increased in resistance, which causes RasPi's 5V as measured between TP1 and TP2 to fall too low. You do need a multimeter to diagnose these problems.
3. Iffy solder joint which has failed after power cycling. This usually requires returning RasPi for an exchange.
Many RasPi problems are power supply problems, such as an iffy polyfuse F3 or an iffy Micro USB cable.
when you say "Corrupted SD card from an unclean shutdown (always shut down using a GUI command or the GNU/Linux "shutdown" command).", does this mean even if the pi does not boot, to remove the power, then the sd card? also, does it matter if you "safely eject" the sd card from the computer? i thought it would only affect it if it was writing/using it (the sd).
The pi works by using the graphics chip to bootstrap the OS, residing on the SD card. If the SD card has become corrupt, you will get a red light for a bit, but nothing will happen as the Pi won't boot. To use a Windows PC comparison, the Pi is using the SD card like a hard disk. If the PC hard drive became badly corrupted, all you would get is the BIOS, then nothing (or probably a few error messages). With the Pi, you get a flickering on the LED's then nothing as the boot process is transferred to the SD card, which is sitting there thinking it is a sheep or something.
I'd re-image the SD card as a starting point and see if that allows your Pi to boot.
> don't quote me on the color. not entirely sure if it's yellow.
I'm confused, are you talking about an indicator on the power supply or on the Raspberry Pi ?
I was talking about lights on the Pi itself. If you get no lights on the Pi then very definitely you are not getting power to the board. Either your PSU is hosed, regardless of any lights showing on it, or the power connector on the Pi is damaged.
If this isn't it, and the good advice given above doesn't help, try removing all USB peripherals from the Pi that you don't absolutely need.
Start with just keyboard and mouse. You can put the peripherals back one by one after you get it booting again.
> i thought it would only affect it if it was writing/using it (the sd).
The trouble is that your Linux installation most likely uses a swap file on the SD card. You cannot tell when it will decide to write to the SD card. There is therefore *no* safe time to eject the SD or pull the power while the Pi is running. You may get away with it for a while, but I promise that you *will* corrupt your SD card sooner or later.
Fortunately, it is very rare for the OS to crash to the point it cannot respond to basic commands like a shutdown request so it is highly unlikely you will ever get stuck. A total OS hang is the _only_ time it is OK to pull the power w/o having the OS shut down first.
I login to my Pi from another computer, so I use a terminal a lot. I always use the 'shutdown -hP now' command before I power off. To do the same from the graphical desk top, click on the red button on the bottom right hand corner then choose the Shutdown button (or similar, you have not told us which Linux distribution you're using).
Wait until all LED activity has stopped and you are left with a single, red PWR LED lit on the Pi. Only then is it safe to remove power. All this is no different than Linux on a regular desktop machine, or for that matter a Wintel box (except they pull their own power for you).