Neomail support has been discontued in cpanel as the script will soon be obsolete and support discontinued by cpanel developers on the script.
Emails will remain unaffected and can be accessed using the other interfaces.
To view the same folders in Squirrelmail, login, select Squirrelmail, click folders and subscribe to the folders you used previously in Neomail. Refresh folder list or the page to see the folders listed in the left column.