Looks like you are able to login to Jenkins after that you are not able to see the dashboard, this is may be due to some network issue so please refresh the page again or you might be entering a wrong username and password. In that case, you can recover this password by disabling security.
Please find the below steps to reset admin password:
1. cd /var/lib/jenkins
2. nano config.xml
3. Change "<userSecurity>true</userSecurity>"
4. Restart jenkins again: sudo systemctl restart Jenkins
Then login to the Jenkins UI and set up the admin user with new password and re-enable the security i.e. reverse the config file and restart the Jenkins.