I almost completely lost my sense of smell after catching the suspected coxsackievirus B infection that also triggered my ME/CFS. The loss of smell was variable: on some weeks my sense of smell would return slightly, but then the next week it would more or less disappear again.
After 2 or 3 years, my olfactory capabilities slowly improved, but even 10 years later have still not returned to their original form, so my sense of smell is still a bit weak.
If you want to try an effective treatment for anosmia (the medical name for loss of smell), the over the counter drug
theophylline 250 to 500 mg for 4 to 6 months is often effective. Ref:
1
I tried theophylline on a short term basis (a week or two), and it did noticeably bring back my sense of smell; but as soon as I discontinued, I would lose my olfactory sense again. However, I did not take theophylline on a longer term basis of several months; had I done so, it may have more permanently fixed my anosmia.
Theophylline increases vasoconstriction, so it can be useful for POTS as well (but I would guess it's better for high flow POTS, where there is too much vasodilation in the limbs, rather than low flow POTS, where there is too much vasoconstriction in the limbs — I have the latter).