In any event, this battery thing is a new development after several months of trouble-free usage.
Probably just dead. You could try cleaning the connections or the like, but it's a slim chance that it'll do much. Someone at some point will probably tell you try freezing it, which can occasionally work with Ni-Cad batteries but is pretty much a terrible idea with Lithium based batteries. I didn't bother to google it, but it's almost certainly Li-ion.
I'm not that familiar with what Dell uses for a laptop bios, but it may display some information about the battery, like if it thinks there is one plugged in or not.
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