Warning: Only a single slice of data found. Did you specify valid 'From/To' times?

[Saish Patil]

here is my code. I don't know why this is happening

https://www.edaplayground.com/x/qiWN

[EDA Playground]

There are many compile errors in your design. But I would think the reason why you were getting no waveforms would be because you specified the top entity as "encoder" instead of "encoder_tb". 

Other errors:

- it's '0' not "0"

- rst should be driven by a "signal assignment operator" <=, not a "variable assignment operator" :=

- your simulation runs forever because of the "clocking" process. You're going to need some way of preventing that (eg "while some_signal loop" instead of "while true loop" OR the VHDL2-008 stop/finish procedures).