The shakai on the back of a mezuza got smudged & is practically unrecognizable. What should be done?
This does not invalidate the Mezuzah. However, if the Name is smudged to a degree that it cannot be read, one must correct it.
In addition, smudging is usually a sign that the Mezuzah has been exposed to damp and moisture, and it is prudent to take it down and check the writing on the inside.
The ShulchanAruch (Yoreh De’ah 288:15) cites that the custom is to write the name Shakkai on the back of the Mezuzah. The custom is mentioned by the Rosh (Mezuzah 18) and by the the Mordechai, and it is clearly an ancient custom, which the BeisYosef writes must be adhered to.
Therefore, although a Mezuzah that does not have the name Shakkai written on the back remains kosher, if the Name is smudged to a degree that it cannot be read it is certainly a bedieved situation, and it must be corrected.