Our mongodb server was running for the last two months without any error. Suddenly it crashed and below is the log.

2017-08-11T13:41:41.320+0100 E STORAGE [thread1] WiredTiger (13) [1502455301:320924][1281:0x7f11bce8e700], log-server: /var/lib/mongo/journal: opendir: Permission denied
2017-08-11T13:41:41.320+0100 E STORAGE [thread1] WiredTiger (13) [1502455301:320991][1281:0x7f11bce8e700], log-server: dirlist journal prefix WiredTigerPreplog: Permission denied
2017-08-11T13:41:41.321+0100 E STORAGE [thread1] WiredTiger (13) [1502455301:321013][1281:0x7f11bce8e700], log-server: log pre-alloc server error: Permission denied
2017-08-11T13:41:41.321+0100 E STORAGE [thread1] WiredTiger (13) [1502455301:321021][1281:0x7f11bce8e700], log-server: log server error: Permission denied
2017-08-11T13:41:42.000+0100 I - [ftdc] Assertion: 13538:couldn't open [/proc/1281/stat] errno:13 Permission denied
2017-08-11T13:41:42.000+0100 W FTDC [ftdc] Uncaught exception in 'Location13538 couldn't open [/proc/1281/stat] errno:13 Permission denied' in full-time diagnostic data capture subsystem. Shutting down the full-time diagnostic data capture subsystem.
2017-08-11T13:41:50.006+0100 I NETWORK [initandlisten] connection accepted from 127.0.0.1:45196 #205521 (23 connections now open)
2017-08-11T13:41:50.006+0100 I NETWORK [conn205521] end connection 127.0.0.1:45196 (22 connections now open)
2017-08-11T13:42:36.004+0100 E STORAGE [thread2] WiredTiger (13) [1502455356:4361][1281:0x7f11bb68b700], file:WiredTiger.wt, WT_SESSION.checkpoint: WiredTiger.turtle: fstat: Permission denied
2017-08-11T13:42:36.004+0100 E STORAGE [thread2] WiredTiger (13) [1502455356:4466][1281:0x7f11bb68b700], checkpoint-server: checkpoint server error: Permission denied
2017-08-11T13:42:36.004+0100 E STORAGE [thread2] WiredTiger (-31804) [1502455356:4480][1281:0x7f11bb68b700], checkpoint-server: the process must exit and restart: WT_PANIC: WiredTiger library panic
2017-08-11T13:42:36.004+0100 I - [thread2] Fatal Assertion 28558
2017-08-11T13:42:36.004+0100 I - [thread2]

***aborting after fassert() failure

please tell me why it is crashed. We are running mongodb version 3.2.