The short answer
In most states the board can approve some special assessments on its own. The limits come from two places: your state's law and your association's governing documents (the declaration, also called the CC&Rs, and the bylaws). When the two differ, the stricter rule usually applies.
Many states let the board act alone up to a set percentage of the yearly budget, and require an owner vote above it. Most also allow the board to act without a vote in a genuine emergency, or when the law forces a repair.
Repairs vs upgrades
The line that matters most is often between repairs the association must make and improvements it chooses to make. Required structural and safety repairs are usually the board's job. New amenities, renovations and upgrades are more likely to need owner approval.
If your assessment mixes the two, ask the board to break the amount down.
What to do if there was no vote
Ask in writing which section of the governing documents and which state law the board relied on. Request the minutes of the meeting where it was approved and the notice of that meeting.
Keep paying, or ask for a payment plan, while you ask. Not paying usually leads to late fees, interest and a lien, even if the assessment is later reduced.
General information, not legal advice. Rules vary by state and by your governing documents.