MyISAM recover updated row
Unfortunately the only way to recover your data is to recover them from a backup, you can import just that single table and then insert the particular row.

I will edit my answer to get more precise here, first of all you write that you use dynamic row length. This makes the recover operation way harder if not impossible compared to fixed row length. If you are seriously want to recover, I would suggest that you first of all get known to MyISAM dynmic data file layout, a good entry point would be here: MyISAM Dynamic Data File Layout. Therefore you have a chance that you can recover, but not with a tool which is written for fixed length files. Unfortunately I am not aware of such a tool (the ones i tested didn´t satisfied my requirements). 

And btw, I think you will have heard or read that more than enough already, but i want to stress this out again: always backup, test the backups, rinse and repeat.