|I doubt Dell have done anything - your computer has developed a fault,|
Tap F8 key during startup and see if you can see a menu with Safe Mode mentioned. If it is there go for it and let us know if it then boots up. Note that in Safe Mode things will look rather different (large icons etc).
If this fault came on recently and you can get into Safe Mode then you could do system restore to a time before the problem came along - type System Restore in search.
Always pop back and let us know the outcome - thanks