If you made a "System Repair Disc" earlier, you can boot from that and choose "Startup Repair". If that doesn't fix it, boot from it again and choose "System Restore", then choose a restore point dated earlier than when the problem started.
If you didn't make one, get on a different Windows 7 PC or laptop if possible and make one on that: Control Panel > Backup & Restore > Create system repair disc
If you use a different PC, it must be running the same Windows "bit" version as yours (ie 32-bit or 64-bit). It does not matter which edition it is (eg Home Premium, Ultimate etc).