Ok, we must first check, if the Power Adapter, is working, and working correctly. You need to take out the battery, now plug in the Wall adapter, and see if it works, if it doesn't then The adapter may be faulty, you really need to check with a Volt meter. Now if the PSU IS OK, then you have a problem most likely with your internal Power board. This needs to be repaired. Now IF it works OK, without the battery, you may find the battery itself has gone faulty. But again, IF battery & PSU, are OK, then the problem is internal. You will need to get this attended to. take it to a Dell service center,and ask for a "Quote" for any repair.
http://support.dell.com/support/topics/global.aspx/support/dsn/document?docid=336966
This link above, will show you how to do a factory restore, in the event, that it's a Software problem and NOT a broken laptop.
If none of that works, then you may have to perform a Reinstall of a Windows operating system. You would need to get a Windows install DVD, set PC to Boot from DVD, in BIOS, and boot off the windows DVD, and install a fresh copy.