Whether it's a burst pipe, a backed-up drain, or spring melt coming through the wall, the first two days after water gets into a basement decide how big the repair will be. Water keeps wicking up drywall and into subfloors, and damp materials can start growing mould within a couple of days.
Start with safety. If water is near outlets, the electrical panel, or appliances, don't walk in until the power to that area is off. Then stop the source if you can: shut the main water valve for a burst pipe, or direct water away from the house if it's coming from outside.
Before moving anything, take photos and video of the water level, the damaged items, and the source. Your insurer will ask for them, and they're hard to recreate later. Call your insurance company early so the claim is open while the evidence is fresh.
Then get the water out and the air moving. Remove standing water, pull out wet rugs and cardboard boxes, and open the space up. Surface drying isn't enough, though: drywall, insulation, and baseboards that stayed wet usually have to come out, because moisture trapped behind a wall is where mould starts.
A restoration contractor measures moisture inside the walls and floors, removes what can't be saved, dries the structure with the right equipment, and rebuilds once readings confirm everything is dry. Closing the walls before that point is the most common mistake, and the most expensive one.