Sounds like the com port configuration of the GPS unit and Arcpad are off. In Arcpad under the GPS preferences you can search for the ports and see if the program can find it. (see here: ArcPad Help) If it cannot find it, you'd have to provide more information on the type of unit you are using to check the hardware settings to see if they are set correctly.
But as a general rule of thumb if it's not working, reboot the device. I used to manage and troubleshoot hand-held data collectors and a large portion of the time where errors were being displayed a good ol' fashion reboot would solve the issue as well.
Cheers