First have you tried restarting the tablet and also the modem/router? Sometimes the fix is that simple.
If that doesn't help try the following.
Make sure that there are no apps that were recently installed, or updated, that could be causing it to disconnect. Either through just messing with the connection (yes some are just made to annoy) or by trying to do things through the WiFi and it keeps kicking it out. So I would check the device.
If that isn't it, then sometimes you need to adjust the Wi-fi sleep policy on the tablet. It can often cause this.
To check to see if this is the issue...
1. On the home screen of your tablet go to "Menu" then "Applications".
2. Next go to "Settings" and finally "WiFi Settings".
3. On this...