What is the command to find the path of a file if we know the file name and the root directory where the file resides..

For eg. if a file abc.dat resides in /home/mydir/myfiles/. I am looking for a command which will be fired from / directory, takes abc.dat as input and display the path of abc.dat as output

Try this :

find / -name 'abc.dat'

It works. But it also returns many other lines not at all related to the filename which I am giving as input
for eg: if i give command as find /home -name abc.dat it returns the output as given below. Many unwanted file names in the output

find: <filename>: Value too large to be stored in data type
find: <filename>: Value too large to be stored in data type
/home/mydir/myfiles/abc.dat - (This is the only info what I want )
find: <filename>: Value too large to be stored in data type
find: <filename>: Value too large to be stored in data type
find: <filename>: Value too large to be stored in data type
find: <filename>: Value too large to be stored in data type
find: <filename>: Value too large to be stored in data type
find: <filename>: Value too large to be stored in data type
find: <filename>: Value too large to be stored in data type

In the above output all lines except one which i have indicated are unwanted. <filename> does not even match or has any similarity with abc.dat

Why is it so?

Find is reporting an error. Just redirect stderr to /dev/null to ignore these if you don't care about them:

find / -type f -name 'abc.dat' 2> /dev/null