target disk mode stopped working

I have a 2015 MacBook with a single USB-C port on which I accidentally deleted a sparse image file. It is running 10.15.7. I put it into Target Disk Mode and connected up to my MBP running 11.5.2. I ran the free version of StellarDataRecovery and could see that the file was there and apparently intact. As it was too large to restore with the free version of Stellar, I rebooted the MacBook and tried using the mv command in Terminal to undelete the file. That did not work. I went back to Stellar and started the the scan all over again, putting the MacBook back in Target Disk Mode, but for some reason it was taking several hours to do the scan. I had to put the computers to sleep so that I could move locations. When I got to my new location and woke up both computers, the external drive had disappeared on the MBP. I have tried rebooting and restarting Target Disk Mode, changing ports on the MBP, using a different cable and so far cannot get the the MBP to recognize the TDM on the other computer. I am perplexed as to why it worked before and now won't. Did putting the computers to sleep cause some kind of problem? How else can I get the MBP to mount the other computer in TDM? Or any other suggestions for how to recover that sparse image file? It is 25 gig.