What can be done when a startup has proceeded to build a product, without conducting a proper market research.

I would start with immediate validation research - interview existing users and analyze their actual usage patterns. This gives you real data about how the product is being used versus intended use.

Next, conduct quick competitive analysis to identify gaps and opportunities. Use these insights to make targeted product adjustments rather than major overhauls.

While not ideal to research after building, it's better than continuing without market understanding. The key is to be agile with changes based on findings while minimizing resource waste. Consider this a lesson learned and build research into future development cycles.