The first thing I would try given what you have said so far is SFC (System File Checker), this attempts to repair faulty or corrupted Windows files. To do this -
Can you click on the 'Start' button ? If you can, type either 'command' or merely 'cmd' into the search box (ALL typing without the ' ' ). This should return a fiile 'command.exe' at the top of any list. RIGHT click on this and select 'Run as administrator'. This will produce a small black coloured Window with white text which ends in a blinking underscore, the cursor. Type 'sfc /scannow' (Note there is a space between sfc and /scannow) and press 'Enter'. This will start SFC running. Let it run until it completes, this can take up to an hour, and at the end it will produce a report which will essentially say one of three things -
1 No file errors were found.
2 File errors were found and repaired
3 File errors were found and not fixed.
If either 1 or 2 then see if your system is working properly again. If 3, repeat the process. It can take two or three attempts to fix all possible errors.
If you cannot get to 'command.exe using the above method then try it in 'Safe mode'. Normally on a Win 7 computer you get to safe mode by pressing the F8 key several times while the BIOS screen is showing immediately after you switch on. This is the screen with the maker's name over it. Pressing F8 will take you to a screen, again white text on a black background, where you will have several options. Choose 'Start in Safe Mode with command prompt. Once you get to the prompt type 'SFC /scannow' as above and see what happens.