Oracle/PLSQL: ORA-00018 Error

Error:
ORA-00018: maximum number of sessions exceeded.

Cause:
You tried to execute a statement that requested a resource. Since the maximum number of sessions have been reached, Oracle will not process any new resource requests.

The options to resolve this Oracle error are:
    You can wait a few minutes and try to re-execute the statement(s).
    You can shut down Oracle, increase the SESSIONS parameter in the initialization parameter file, and restart Oracle.