Stroke or Transient Ischemic Attack (TIA): Given the acute onset of weakness and decreased range of motion (ROM) in one hand without trauma or pain, a vascular event affecting the brain is highly plausible. The absence of pain and the specific involvement of one hand suggest a neurological cause, with stroke or TIA being at the top of the list due to their potential for sudden onset and focal neurological deficits.

Peripheral Nerve Injury or Entrapment: Conditions like carpal tunnel syndrome or ulnar nerve entrapment could cause weakness and decreased ROM in the hand. Although these typically present with pain or paresthesia, some cases might initially be painless.
Radial or Ulnar Neuropathy: Direct compression or injury to these nerves can lead to specific patterns of weakness in the hand, though usually, there's some history of trauma or compression.
Musculoskeletal Issues: Conditions such as tendonitis or a sudden onset of arthritis could potentially cause weakness and limited mobility, though pain is typically a significant component.

Spinal Cord Infarction or Compression: Though less common, spinal cord issues can present with acute weakness. Given the potential for severe and permanent disability, these diagnoses must be considered, especially if there are any symptoms suggesting spinal cord involvement (e.g., bilateral symptoms, bladder/bowel dysfunction).
Multiple Sclerosis or Acute Demyelinating Neuropathy: These conditions can present with sudden neurological deficits, including weakness. They are crucial to identify early for appropriate management.
Infections like Abscess or Osteomyelitis: Though typically painful, in immunocompromised patients or those with certain conditions, infections might present atypically, making them important not to miss.

Neoplastic Conditions: Tumors affecting the spinal cord, peripheral nerves, or the hand itself could cause progressive weakness but are less likely to present acutely without other symptoms.
Mitochondrial Myopathies or Other Metabolic Myopathies: These rare conditions can cause muscle weakness but usually have a more chronic course and additional systemic symptoms.
Acute Intermittent Porphyria: A rare genetic disorder that can cause acute neurological symptoms, including weakness, though it typically includes abdominal pain and psychiatric symptoms.