this is very likely not so much MaxSea as a GPS driver issue. Are you using a USB GPS by any chance?
In any case, either with USB or with real serial
port it may be the case with some drivers that the device needs to be "closed and restarted" after computer goes to sleep. Since the navigation
program does not know computer went to sleep (nothing really does) it can't (or won't) do that resulting in no data on a serial
On the one hand it may be worth playing with port settings (in case it is a driver issue) though I would suspect that won't help.
On the other, you may have more success replacing true "sleep" with a different power saving mode - i.e. let computer turn off display and shut down hard drives, but do not put it into actual sleep mode.