Hi, Happy here from the VBA (Valheim Builder's Association) pty ltd. Sounds like one of the lower supports has given way or something, maybe a floor piece? did you make any alterations to your structure while building the second level? Did a Grey Dwarf come in & clobber your home? maybe you moved or changed a support pole somewhere?

When building, hover your hammer over the built piece you just placed and watch the colour of the piece change. If it turns red, it needs support of some kind. Recommend using core wood until you unlock the much more sturdy VBA approved iron beams & stone pillars.

maybe a screenshot of what you are building would help find the issue. Good luck either way.