直到20天前，我一直使用R包mice。今天，我尝试加载library(mice)，看来我已经没有安装它了。因此，我只运行install.packages('mice')并收到以下错误：
> install.packages('mice')
Warning in install.packages :
unable to access index for repository https://cran.rstudio.com/src/contrib:
internet routines cannot be loaded
Warning in install.packages :
unable to access index for repository https://cran.rstudio.com/src/contrib:
internet routines cannot be loaded
Warning in install.packages :
package 'mice' is not available (for R version 3.5.1)
你知道我该如何解决这个问题？