Hi Lorenzo,
Please provide the following information in context of this issue:
What is the Model of the Dell laptop?
This issue usually occurs when there are changes in the BIOS settings. You can refer to the link for reference:
http://en.community.dell.com/support-forums/laptop/f/3518/t/19491654.aspx
**Note:**Modifying BIOS/ complementary metal oxide semiconductor (CMOS) settings incorrectly can cause serious problems that may prevent your computer from booting properly. Microsoft cannot guarantee that any problems resulting from the configuring
of BIOS/CMOS settings can be solved. Modifications of the settings are at your own risk.
I would suggest you to post the query on Dell community to get the details on this issue. You can refer to the link to do so:
http://en.community.dell.com/support-forums/laptop/default.aspx
For further help, you can also contact Dell support. Use the link:
http://www.dell.com/support/contents/us/en/19/category/Contact-Information?c=us&l=en&s=dhs&cs=19
Do let us know if you need any other assistance on issues with Windows.