First thing is to check with your ISP to make sure 1/. they are not doing any maintenance in your area, and 2/. they should be able to do a line check to see if there is anything wrong at their end. If everything checks out, refer to your monthly data limit and see if 1/. you are still within your allowance, and 2/. the bill has from last month has been paid.
Now it's all at your end. Turn off the modem for 30 minutes. If you have a router, turn that off as well. This is the perfect time to think of a new password, and if you know how to change your modem's password, do it and do not tell anyone at this stage, not even family. If the kids start whinging and saying they have an assignment that needs to be handed in tomorrow, tell them they should have thought about that earlier. You also get to test the set-up with only one computer using it.
If the problem is now solved, it means too many others were using the set-up, forcing it to shut down. Now you can bring in other computers and devices slowly, one at a time, and watch the performance change.
Other than that, maybe it's time for a new modem!