On start up (before Windows loads) keep tapping either F5 or F8 (be aware that some manufacturers use F8 for system recovery!) then use arrow keys to highlight 'Safe Mode with networking' and hit Enter/Return, click on a user account, enter the Administrator password (if you don't know it, there probably isn't one so leave it blank) and hit Enter/Return.
Or as a last resort...
(The first one may not work for all models but worth a try).
On start up (before Windows loads) press F8 and choose the option “Repair your Computer”
***This will erase all your unsaved data!***
Or
1) Hold down the power button until it shuts down.
2) Press and hold the zero key and tap the power button once.
3) When you hear beeping release the zero key.
4) Click Yes to continue system recovery.
5) Click 'Recovery of Factory Default Software', click Next.
6) Click 'Recover to out-of-box state', click Next.
7) Click Next to Start recovery.
***This will erase all your unsaved data!***
© arou42