I would suggest you to press F12, it would proceed to boot menu where in if you have an option of "Diagnostics", proceed to run the same which should find the problem along with the error message. If "Diagnostics" is not mentioned, select hard drive and press enter, ideally it should detect the operating system and boot or might restart and then detect the operating system. If it still doesn't, its a clear indication of the NT detect on the operating system which has gone bad. In this case, please reload the operating system on the computer.
Kindly revert for further clarifications,
Thanks and regards,
Dell Social Media Responder.