Below are listed reasons for Resource Plans are not being loaded or Workbench is hanged.

1. Check Allocation Board filter is defined correctly.
2. Check if the Dates filters or State filters (Planned / Requested / Confirmed / Allocated) are checked correctly on the workbench.
3. Any of the resource plan (which has to be listed) dates are not populated or having dates of huge duration (like 100 of years). (Please refer this KB Article)
4. Check if entity (Project / Demand) date's are populated correctly. (Please refer this KB Article)