The symptoms you list are indicative of a dead battery.
I'm unfamiliar with the specifics of the Compaq portable 286, but I can't see why it would be very different to other 286 based PC compatible computers.
And so the steps to get your machine working again are basically:
Step 1: Confirm dead battery - replace if so.
Step 2: Identify make/model of hard drive and provide that information to this forum. Forum members will tell you the drive type number.
Step 3: Run a setup program (sometimes specific to the computer) specifying:
* Present date/time
* Type of floppy drive/s (eg. 360K/1.2MB/720K/1.44M)
* Hard drive type number (eg. type 2)
BATTERY
You are going to have to open the computer to check the battery. It will possibly look like one of the following two, which are very popular in 286 based computers:
http://members.dodo.com.au/~slappanel555/misc/BATT_1.JPG ( 3.6 volts)
http://members.dodo.com.au/~slappanel555/misc/BATT_2.JPG ( 6 volts with series resistor)
Replacements are available. For example, eBay item 230206899317 ($4 each) can be used to replace the two types above.
HARD DRIVE
While you're inside the computer, see if there is a hard drive present. If so, note it's make/model and pass that to us then we'll tell you the drive type number.
SETUP DISK
As for the setup program/disk, I've found an earlier thread on these forums where someone provides a link to the disk (in the second post).
Here's a link to that thread:
http://www.vintage-computer.com/vcforum/showthread.php?t=11808