I removed Tacacs from my router config by "no aaa new-model" but I am not anymore able to login by the username and password that I had already configured on my router.
How can I activate this username and password?
You need to re-enable aaa new-model.
You need this command to be able to use your local username and password command. You don't need to run TACACS on your routers. You can use LOCAL authentication. I think that is what you are trying to do anyway.
Right now your router is local for only the password on your vty lines and enable password.
Most likely you didn't set up a password for your "line vty" so you will have to console into the equipment.