Laptops

If the Dell is still under warranty it is a definite case for return/repair.

The AC socket on the Dell is one possibility, the connector to the motherboard may have a break in it. This problem usually only shows up if the laptop has suffered a trauma or has been dropped while the AC cable is attached.

If it is the AC adaptor it will have to be replaced. Perhaps you could get access to another adaptor in order to rule it in or out of the equation.

If, as you have said, it has sometimes not come on at all, that only confirms one of the two possibilities above since the battery may have become so depleted as to prevent normal startup until normal current-flow was resumed.

Whatever the cause, a trip to a Dell or other computer technician would be the best course of action.

We learned at the university I was employeed at that it was the adaptor(wrong one) and the laptop wasn't getting enough power to keep the screen on bright, while charing the battery and so forth.