Recently I've got a notification which said: "Agent has stopped monitoring. The following errors are reported : COLL_DISABLED|DISK_FULL". This message is pretty clear and straightforward. ORACLE_HOME for agent is located in /opt mount point.

[oracle@sol01 ~ ]$ df -h | grep opt
rpool/OPT              120G    109G        11G    99%    /opt

After the cleaning in order to make more space (note: default value for UploadMaxDiskUsedPct is 98 percent) agent usually resumes operations but this time it has remained in disabled state even if disk had sufficient amount of space (32 GB).

[oracle@sol01 ~ ]$ df -h | grep opt
rpool/OPT              120G    67G        32G    68%    /opt

[oracle@sol01 ~ ]$ emctl status agent
Oracle Enterprise Manager Cloud Control 12c Release 2
Copyright (c) 1996, 2012 Oracle Corporation.  All rights reserved.
---------------------------------------------------------------
Agent Version     : 12.1.0.2.0
OMS Version       : 12.1.0.2.0
Protocol Version  : 12.1.0.1.0
Agent Home        : /opt/oracle/agent12c/agent_inst
Agent Binaries    : /opt/oracle/agent12c/core/12.1.0.2.0
Agent Process ID  : 22454
Parent Process ID : 22163
Agent URL         : https://sol01.local:3875/emd/main/
Repository URL    : https://cloud.local:4903/empbs/upload
Started at        : 2016-02-08 17:59:34
Started by user   : oracle
Last Reload       : 2016-02-16 14:24:27
Last successful upload                       : 2016-02-16 14:14:40
Last attempted upload                        : 2016-02-16 14:14:40
Total Megabytes of XML files uploaded so far : 42.78
Number of XML files pending upload           : 0
Size of XML files pending upload(MB)         : 0
Available disk space on upload filesystem    : 1.30%
Collection Status                            : [COLLECTIONS_HALTED(
  UPLOAD SYSTEM Threshold (UploadMaxDiskUsedPct: 98) exceeded with 98.69987)]
Heartbeat Status                             : Ok
Last attempted heartbeat to OMS              : 2016-02-16 14:26:39
Last successful heartbeat to OMS             : 2016-02-16 14:26:39
Next scheduled heartbeat to OMS              : 2016-02-16 14:27:39

---------------------------------------------------------------
Agent is Running and Ready

In this case solution is simple and you don't have to restart agent, you can manually invoke upload instead...

[oracle@sol01 ~ ]$ emctl upload
Oracle Enterprise Manager Cloud Control 12c Release 2
Copyright (c) 1996, 2012 Oracle Corporation.  All rights reserved.
---------------------------------------------------------------
EMD upload completed successfully

...and then agent will resume operation.

[oracle@sol01 ~ ]$ emctl status agent
Oracle Enterprise Manager Cloud Control 12c Release 2
Copyright (c) 1996, 2012 Oracle Corporation.  All rights reserved.
---------------------------------------------------------------
Agent Version     : 12.1.0.2.0
OMS Version       : 12.1.0.2.0
Protocol Version  : 12.1.0.1.0
Agent Home        : /opt/oracle/agent12c/agent_inst
Agent Binaries    : /opt/oracle/agent12c/core/12.1.0.2.0
Agent Process ID  : 22454
Parent Process ID : 22163
Agent URL         : https://sol01.local:3875/emd/main/
Repository URL    : https://cloud.local:4903/empbs/upload
Started at        : 2016-02-08 17:59:34
Started by user   : oracle
Last Reload       : 2016-02-16 14:24:27
Last successful upload                       : 2016-02-16 14:29:40
Last attempted upload                        : 2016-02-16 14:29:40
Total Megabytes of XML files uploaded so far : 42.78
Number of XML files pending upload           : 0
Size of XML files pending upload(MB)         : 0
Available disk space on upload filesystem    : 32.53%
Collection Status                            : Collections enabled
Heartbeat Status                             : Ok
Last attempted heartbeat to OMS              : 2016-02-16 14:29:39
Last successful heartbeat to OMS             : 2016-02-16 14:29:39
Next scheduled heartbeat to OMS              : 2016-02-16 14:30:39

---------------------------------------------------------------
Agent is Running and Ready