Cliff Jacobson advises placing your groundsheet protector inside the tent - this avoids any leakages reaching you.

I had the same problem with my coleman viper. Pretty much the same process as john says, except I cleaned mine off and then sprayed the bottom of it with Fabsil when it was dry. The silicon spray looks a little strange inside but so far no water has come through it.

Same way you'd re-proof the fly sheet of a tent.

1. Turn the tent upside down to fully expose the underside of the groundsheet.

2. Treat for mildew if necessary, then give it a good clean by sponging with a non soap product like Nikwax Tech Wash.

3. As you do it, give a good inspection for tears or little holes no matter how small - repair as necessary with either Goretex patches or repair strips available from some tent manufacturer's. Sponge with clean water to rinse.

4. Re-proof by spraying with Nikwax Tent & Gear Proof while it's still wet, following product instructions.

5. Allow the underside of the groundsheet to air dry completely, wiping excess proofer off with a damp cloth.

6. If the tent has taped or sealed seams, erect the tent and retape the seams. If you don't still have the kit that might have come with the tent, you can buy the tape and sealant at many outdoor or camping shops or online from your tent manufacturer or other outlets.

I re-waterproofed  a 4 person sized tents flysheet and groundsheet base this way and needed a 500ml container at £5.99. Can't guarantee success because I haven't seen your tent but if the groundsheet is undamaged and just needs re-proofing this should work. But if the groundsheet is worn or damaged then the repairs will be needed or you might be better advised to re-invest in a new tent. Other brands also have products you could use instead of Nikwax but I mentioned their products because I use it the most.

I have the same problem with my Vango Hurricane which is at least 15 years old. As I cannot get the groundsheet replaced I use my orange survival bag. You can either put it under the tent or use it inside.