working through a list of dat on a sheet. Allocated limited info to staff via userform

hi guys i have a sheet full of data. This has been allocated to staff as a worklist but there is chance amember of staff could corrupt it.

I am trying to create a userform where by somebody selects their name from a dropdown list and it brings up the details of the next unworked case. the user then fills in the details and press submit it then poulates on the backsheet saves and moves to the next case

The data is standard excel spread sheet no tables with each case staying on its allocated number and moving horizontally through the letters

E.g case 1's data is a1 b1 c1 d1 etc

So i am looking for the vba code that would link these cells and move onto the next case

Hope someone can help me

Thanks in advance

I have attached a workbook.

if John wanted to work his accounts he would select his name and lauch the second userform.
it would then bring up the details of a the first empty case. he would work it fill in the userform and submit this would populate the sheet and then bring up the next case that has not been worked with his name attached to it.