You'll need to do the following
1- File an F.I.R without any further delay
2- Goto the ETO Office submit an application alongwith the F.I.R with a copy of your lost registration book (Documents required for the submission you can easily ask a tout or if you can find a cooperative person at the information counter, they can give you a check list)
3- You will receive a duplicate book after sometime.
Hope this issue of yours get resolved quickly