Lack of motivation to carry out research or search for positions post PhD. How to address this feeling? Tl;dr: Defended my PhD two weeks ago. However, right from the time of the defense and up until the present moment, I am experiencing a huge lack of motivation and sense of purpose to do anything related to research or search for future positions. How to address this?

Other details****: I had applied for other positions before my defense and was not successful in securing a suitable position. I have an offer from my PhD advisor about a postdoc position which I am expected to start early next year.

Leaving every other concern aside, I am very appalled by my lack of motivation and the constant feeling of lethargy to start with the pending research work and/or resume with the dedicated job search.

I have no idea how to address this feeling. I have taken two hiking trips during the last two weekends. However it feels empty and unproductive inside. I have so much to do before I start with a postdoc position. But all am I doing is wasting time.
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.