Brief Answer:
More information is needed.

Detailed Answer:
I read your question carefully and I understand your concern.

In order to properly evaluate the issue it would be useful if you could provide some more information such as:

- is the weakness of a sudden onset or has it progressed gradually over weeks/months.
- does the weakness involve only the hand, or is there accompanying weakness in the arm, shoulders, face muscles (facial asymmetry), lower limb
- is there visible muscle atrophy, muscle wasting in the hand
- does the weakness involve all fingers or only some of them, is it for all movements or only for certain movements, such as hand or finger flexion or extension
- any other signs such as headache or neck pain, nausea, vomiting, balance issues.

Based on the provided information, if the deficit involves only the hand or fingers I would say there is an issue of the peripheral nervous system, such as peripheral nerve compression (such as radial nerve) or nerve compression in the neck from a herniated disc. If it is a gradually progressive issue with muscle atrophy motor neuron disease (ALS) must also be considered.
If on the other hand there is involvement of the face or lower limb as well, with an acute onset then a stroke must be considered and you should head directly to the ER.

I remain at your disposal for other questions, but possibly with some more information on the questions I asked, in order to make a more precise evaluation because the causes of weakness in the limbs can be many.

---