If you go to the Dell support site online and enter your service tag # , then search 'boot problem' or 'startup problem', you'll get specific troubleshooting instructions for your laptop.
Otherwise,
here's a general troubleshooting article that may help you to diagnose the problem, and
here's a somewhat dated but still very valid boot troubleshooter.
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
If I've managed to answer your question or solve a problem, please take just a moment to rate this post....thanks!