I would check the base first. When mine sagged i found my kids had jumped on the bed and broke the base. If you have no base your springs may be broken.
I am not sure you can fix a mattress that is sagging in the middle. If it is sitting on a good solid box spring and the frame holding the box spring and mattress is not cracked or broken then it must be the mattress. If you have a queen or king size bed it does help to have some support under the middle rails of a frame. A block of wood or some bed risers for support from frame to floor. If your mattress is more than 6 or 7 years old it might be time to buy a new one.
I had one too that was sagging in the middle and it turned out that one of the supports in the middle of the frame broke. You can check that and replace if needed.
I have a plastic chair mat under my computer desk that the edges are curled and won't lay flat. Have been unable to bend them back as the plastic is hard
What type of a mattress is it - coil, inner spring, latex foam, ?
How old is the mattress?
How is it being supported - on a box spring, on a platform or frame, or is it on the floor?
I would check the base first. When mine sagged i found my kids had jumped on the bed and broke the base. If you have no base your springs may be broken.
I am not sure you can fix a mattress that is sagging in the middle. If it is sitting on a good solid box spring and the frame holding the box spring and mattress is not cracked or broken then it must be the mattress. If you have a queen or king size bed it does help to have some support under the middle rails of a frame. A block of wood or some bed risers for support from frame to floor. If your mattress is more than 6 or 7 years old it might be time to buy a new one.
Go shopping 🛏
I had one too that was sagging in the middle and it turned out that one of the supports in the middle of the frame broke. You can check that and replace if needed.
Check the supports. It probably needs replaced though.