Lack of motivation to carry out research or search for positions post PhD. How to address this feeling?
Give yourself some time

You just finished an important step in your career, which has presumably taken you between 3 and 7 years. Congratulations! For a while, it is normal to feel like celebrating your achievement instead of going on with everyday research and making important decisions. It is even more normal to have doubts about the next step.

Try to find out why you are not motivated

The good news is that you have an offer for a postdoc position, so (assuming you do not have to give an amswer now) you have some time to reflect on why you are not motivated. Is it just a temporary feeling? Do you really want to continue in academia? Do you want to do research at all? The answers to these questions should become clearer after a while.

Talk to people

Your advisor might have some suggestions about other options you might have after your PhD that motivate you more. You can also ask other colleagues/people in the field that you trust. Maybe they have had a similar experience and can tell you how they overcame it.