First, make sure you are trying to connect to the correct WiFi. You would be surprised how many people try connecting to say a neighbors which has a similar name, etc.
Second, make sure that you are connection to the right version of your WiFi. Many modems/routers now have both 2.4Ghz and 5.0Ghz connections. While they can have the same name and password, if set up that way, often they will have different names and different passwords.
Third, if you have your WiFi connection hidden (so that others can't see that it is even there) you may need to try unhiding it. Some devices do not connect well to hidden networks.
Fourth, again, check the password. Are there supposed to be any capitals? Any numbers? Symbols? Could someone else have change the password? It happens.
NOTE: If you put any 'spaces' in your password, I suggest you go back into the modem's/router's settings and remove it/them. There should never be blank spaces in a password.