I was trying to load software for a modem but forgot the admin password. I then tried to reset the system password using the terminal, now i can't login at all. How can i bypass this to at least be able to log on again then fix things from there.
Another reason why your password might not be recognized is that the keyboard layout (input source) has been switched without your realizing it. At the login screen, you can cycle through the available layouts by pressing the key combination command-space or command-option-space. See this support article.
You must back up all data before continuing, unless you've already done so. If you need to back up but can't log in, ask for instructions.
Otherwise*, boot into Recovery mode. When the OS X Utilities screen appears, select
Utilities ▹ Terminal
from the menu bar. In the Terminal window, type this:
Press the tab key. The partial command you typed will automatically be completed to this:
Press return. A Reset Password window opens.
Select your boot volume ("Macintosh HD," unless you gave it a different name) if not already selected.
Select your username from the menu labeled Select the user account if not already selected.
Follow the prompts to reset the password. It's safest to choose a password that includes only the characters a-z, A-Z, and 0-9.
from the menu bar.
You should now be able to log in with the new password, but your Keychain will be reset (empty.) If you've forgotten the Keychain password (which is ordinarily the same as your login password), there's no way to recover it.
*Note: If you've activated FileVault, this procedure doesn't apply. Follow instead these instructions.