It is, unfortunately, one of the typical situations in Indian companies because of the high attrition rate.
In this case, you need to do the Exploratory Testing of the product. It is about exploring, finding out about the software, what it does, what it doesn’t do, what works and what doesn’t work.
In this testing, you will come to know about the system and its basic workflow. In Exploratory Testing, you can also discover ‘blocker’ bugs that are harmful to your system and therefore, protect your system from being crashed.
If you are a white box tester, the next step is to look for different module codes. By this, you will be able to analyze the test cases for different modules and their relations.