Why isn't my SD Card working after rooting?

[Hasibuzzaman Tonmoy]

I own a LG G4 H818P which came with MM 6.0 installed, I downgraded it to 5.1 and then rooted it. The root process is successful but my SD Card has stopped working on this phone. When I insert the card, it shows that "SD Card is blank or has unsupported file system" but the card is working fine on my PC or other phones.