this happened sometimes, the wii u's wifi card is just kind of weak and will lose connectivity from time to time. make sure nothing's on top of the console

also the wifi card could have given out, in which case you could either get a replacement (requires disassembly) or buying an ethernet adapter supported by the wii u and getting online via wired connection

suddenly it started working out of nowhere. i have an ethernet adapter right beside me so i'll keep it in my room as a good luck charm