Always try restarting first
The first thing you must do while trying to solve any software problem is to try restarting the computer or just turn it off. Press the power button for five seconds in case the computer has hung. Disconnect the power, wait for a minute and then connect the power and turn the computer back on. This will work most of the time and is the most common trick to get your computer to start working again. It was even a running gag on the IT Crowd TV show.

My computer is running slow
If restarting hasn't worked and your computer isn't ancient, then you might want to start by checking if the problem lies with your computer. If downloads are slow, try using speedtest.net to see if you are getting the right Internet speed. If your programs are running slow, then check your main drive in Windows Explorer, it is usually (C:). If it shows as red, try moving some files from the main drive into other drives or pen drives. Do not touch files in the Windows, Program files and Root directory (aka C:). Files on Desktop are safe to delete. You can use "Disk Clean-up" to clean temp files safely. Search for Disk Clean-up in the search bar next to the Start/Windows icon. You can also uninstall unused programs by Pressing Windows key + R and typing appwiz.cpl.