Excel sheets are potentially very huge. Do you have any idea of how many rows or columns you need to search? You can use read_xlsx() from the readxl package, similar to what FJCC suggested.
library(readxl)
df <- read_xlsx(file_name, range="A1:Z1000") # read excel sheet into dataframe
value <- df[!is.na(unlist(df))] # unlist to convert df to vector, then find non-missing value