Car wont start, any guesses?

clean your battery connections. That would be the first step. Check both battery wires to see if they are too old and spent. Check where the + connects to the coil and where the - connects to the engine.

First of all, be sure your battery is charged; do you have a voltmeter/battery charger to check it? Either that, or replace with a different battery from another car just to eliminate it as a suspect. Once you confirm your battery has power, what happens when you turn the key to "ON"? Do you still have power to your dash/radio, or do they cut out after you crank the key to
"START"? If the latter, then you probably have a bad ground cable or +battery cable. Even if it's not them, it's one of those things that it really doesn't hurt to replace anyway as they go bad with time. Usually if it's the cables, you can wiggle them and get power to come back to your dash at least, but it still may not get the solenoid to engage the starter.
Once you have confirmed the cables are good (or just replaced the cables), you can check the solenoid itself by bypassing it. Simply bridges the positive terminals with a screwdriver (be sure the car is in neutral before you do this). If you can get the starter to crank this way, then obviously your solenoid is bad.
Good luck.