It sounds like he changed the password.  If the does not know what it is, you will need to call Dell and produce proof of ownership.  Then you'll be issued a master backdoor password for your system.

Note:  if you are out of warranty, call the Out of Warranty support line and there is a $50 charge for the support call out of warranty.  In warranty the call is free.

The fix remains the same - if you don't have the password, you must contact Dell support.

Thanks, the problem is that I'm in Venezuela, I have the sales invoice, but in no event occurs that changes the password