All the best.
an antidepressant or cognitive therapy or another psychotherapy depending on your situation. If you have trouble sleeping or your partner notes that you stop breathing for brief periods at night , you could have sleep apnea- a highly treatable condition. The first step is a full history and medical work up, then perhaps a referral.
Are you experiencing any stressors? Has your diet, sleep changed lately? Are you feeling depressed or sad about anything? When was your last physical? Do you have any nutritional issues such as low iron, not drinking enough water, other vitamin/mineral deficiencies? Is it your thyroid? There are so many potential causes. First, I would visit my PCP to have a full physical if you haven't had one lately. Then I'd reflect on any stressors. Usually, the answer to questions like these can be determined with a full medical work-up and an individual taking a really close look at their lives and if there have been any changes that would lead to mental fatigue.
Quite often lack of motivation/focus energy occur along with mood symptoms, such as low mood or anxiety. Poor focus/attention can also be incontext of other issues such as ADHD, however this tends to be more chronic (long-term) and not an issue out of the blue- but nevertheless may be as a possibility.
The next best step is to likely make an appointment with your primary physician and see their recommendations. If you are also experiencing issues with mood/anxiety as well, I would recommend to go ahead and make an appointment with a specialist, such as a psychiatrist for further investigation.