For some reason my Galaxy Y GT-55360T phone running Android 2.3.6, won't connect its SD card to my Ubuntu 12.10-running PC when I plug the USB cable into it. It did before, I have no idea what I did to cause it to not mount the SD. Please help me, this is getting very annoying.
As you did not mention your phone nor the Android version, we can only make wild guesses.
Simplest case: It is de-activated on your Android device
Most likely you unintentionally deactivated the USB Mass Storage (or MTP, depending on device/version) mode. To get it back:
When you connected the USB cable, wait 2-3 seconds, and then pull open the notification area. It should show an entry with the USB symbol which lets you chose the action to perform ("charge only", "USB Mass Storage", "MTP", ...). Chose the one you want. Now your computer should see the new device.
Simple case: Your USB-cable got "broken"
Of course another possibility is your USB cable "got broken" (e.g. due to physical stress). If the above did not solve your issue, you might try with a different cable.
More difficult case: Ubuntu messed up
Third option I can think of is that Ubuntu somehow messed up. To rule this out, try with a different computer (e.g. visit a friend if you don't have a second computer). You can also try booting from some live CD/DVD on your Ubuntu machine. Using the same machine for the test bears the risk that, if it's a hardware issue with the machine, results are simply the same; in case your USB port might be the issue, try a different port (and try a different device on the same port to cross-check).
If it's Ubuntu messing things up, our sister-site AskUbuntu might prove helpful, and we are not any more...
Hard case: USB port on your computer is broken
I already showed above how to rule this out: Try connecting your Android device to a different USB port. If it works there, the former port most likely is broken. If not, try connecting some other USB device (external hard disk would be ideal) to the former port. If that works, the port is OK.
Ultra-hard case: USB port on your Android device is broken
If you cannot get anything to work on your devices USB port, it's a case for the repair shop...