It could be a number of issues.

The CMOS battery has died. The CMOS battery is a battery that maintains the date and the clock and stores the BIOS setup configuration. These batteries last a long time (5+ years), but when they die need to be replaced. It is an easy task. It is simply a matter of locating the battery on the computer's motherboard, removing it and plugging in a new one. On really older laptops it might be a NiCad battery soldered on the system board, which is much trickier to replace.

The system board or CPU might have a problem. An expensive repair if it is an older laptop. The AC adapter might be defective and the battery might be dead or uncharged. The internal power supply might have a problem. The hard drive might have died.

Foner Books: Computer Repair offers diagostic flow charts to help you figure out the problem. Laptops are tough to repair, as they are delicate and unless you know what you are doing you are better or having an experienced laptop tech do the repair. In many case they will your can send it someone for or bring it to a local repair shop and for $50 to $60 they will diagnose the problem and if you choose to do the repair apply the diagnostic fee towards the repair costs.