It's very difficult to describe - but at my worst, no matter how hard I tried, the energy just would not flow through my body to get to my muscles, and getting out of bed was impossible. If you can imagine that every movement you make requires energy to get to the muscles to make them move, then without that, every muscle feels like it's hitting a brick wall - it just seizes up and refuses to contract or expand to enable movement.
This is not just in arms and legs, but I used to get blurred vision when I was tired, and my eyes wouldn't open, (or one would open and one would have to be closed,) tinnitus in my ears, shaking in my hands, feet, arms, legs, head, bowel and gynae spasms and 'freezes', affecting my ability to go to the loo, and of course, my period flow.
I would crawl up the stairs because I couldn't push myself to stand, and come down the stairs on my bum - sometimes I would avoid going to the loo for hours because it was upstairs and I was downstairs. I wouldn't wish it on my worst enemy.
Whether it's a mitochondrial dysfunction that arises from gut bacteria abnormalities, or an autoimmune condition, or a reactive condition to stress on the brain-blood barrier, no-one's really pinned it down yet I think. I found taking D-ribose, Acetyl L-carnitine, a general multi-vitamin, extra vitamin D, eating organic or as 'cleanly' as possible, all helped. I had a course of CBT, which I was skeptical of, but it did really help me cope with the present challenges I had at the time, and think in a more forward-looking way. Pacing can help but only when you take it very very very slowly, at the pace that the person with the condition thinks they can take.
My mum decided that tennis, swimming, and exercise at home was going to help me - it didn't, I just crashed majorly.
However, finding your baseline, and working up from that in tiny tiny increments may help. I.e. if bedbound, then start at sitting up for two minutes and do that for at least a fortnight. It's slow, and it's difficult, but I went from being a wheelchair user to having a full-time office job, without many adjustments. Having my husband and children was a life saver, as they were the ones who saw me the most, and they knew how ill I was, and that I wasn't faking it, as others in the family thought. Having someone believe the pain and fatigue you're in was the most supportive thing they did, as it truly stopped me from going bonkers. It was hard on them all, but we found ways around things, and eventually social services came in and helped for a few years, until I got better.