Always try a Reindex first. Housekeeping menu->reindex / cleanup.
If this does not fix the problem please contact MainPlan support using the link at the bottom of the page.

The session probably crashed or was shut down without exiting MainPlan first and the program did not get a chance to update its table of logged in users.
To reset the user table you must use the Access Control screen.  Log in with the master password (use the contact link at the bottom of the page to request the master password) or log in as another user with administrator privileges. On the Access Control screen you will see a tick box for logged in users located in the top left hand corner (below the Username and Password). Just untick any user that is shown as logged in but shouldn’t be. The down arrow next to the tick box will take you to the next logged in user. Please contact us using the link at the bottom of this page and we will email you instructions of how to use the master password if you cannot get access to the system any other way.

System menu->Utilities->Options->Switches1 and Untick “Perform Automatic Generation of Routine Work Orders”

Main Menu->Routines
Locate the schedule drop down (middle right) and select “Not in Use”

System menu->Utilities->Options->Field Choices
For the Choices for Field Type, ensure the first selection is not R.

For example D,R,W,, is OK for the choices, but R,D,W,, is not OK.

Note: Two commas together (,,) signifies a blank entry and is always required.

Sometimes values (or more specifically one value) entered into a drop-down selectable field (for example work order type) is not available when you try to use the drop down list.
This commonly occurs when the value actually in the field is not one of the values in the list (commonly a blank value) and the value being displayed ‘pushes’ the last entry off the list.
The solution is to select Browse and make a note of all the different values that have been used and the ensure all values used are entered into the possible choices (on the Utilities->Options->Field Choices menu). Please take special note of the blank value (entered as ,, in the field choices).