Why did everything electrical start malfunctioning in my house?
Call your power company and report an outage

When half your 120V circuits are dead and half work -- and the dead half comes back on when you turn on an oven or water heater or dryer (but those things don't work) -- that means one of your hot wires has failed coming from the power company.  It also may be a problem with the main buses or main breaker. 

Now, if your 240V loads like water heater work fine, but lights alternate between too-dim and too-bright/burning out, that's a more serious problem - yes, more serious than an outage.  It is a lost neutral, and it will cause your 120V loads to wobble above and below 120V, which can fry appliances and potentially cause a fire.