SDMount from the market place allows you to remount the card without power cycling the phone. One click on the widget and you card is scanned and remounted.
Just to put a bow around this discussion, it's important to understand that "mounting" your Droid as a drive on your computer essentially makes your Droid completely ignorant about the SD card. And as I discovered awhile back it's very easy to get the order of unmounting (ejecting) and disconnecting from the PC wrong leaving your SD Card in limbo with neither the computer nor your droid able to manage the SD Card.
As others have noted, rebooting (power cycling) your phone should resolve the issue by putting your droid back in charge. Or you can simply write down the correct procedure for mounting/unmounting and do it right. But I find I can't recall where I wrote it down.