Oh no. It sounds like someone in the apartment might have purchased something that is interfering with your wireless signal. This can be some new wireless thing they bought, including a router. Or, a baby monitor.
So I see two choices:
Try all of the difference wireless channels available in the router's configuration utility.
Failing that, try a new router with a very different frequency band. For example, if you have a 2.4 GHz router, then try a 5 GHz router.
The funny thing is, if this is caused by a neighbor's new router, then their signal is probably all messed up too!