After typing dmesg I noticed these errors:
[Thu Sep 17 23:39:52 2015] ata2: SATA link up 6.0 Gbps (SStatus 133 SControl 300)
[Thu Sep 17 23:39:52 2015] ata2.00: supports DRM functions and may not be fully accessible
[Thu Sep 17 23:39:52 2015] ata2.00: supports DRM functions and may not be fully accessible
[Thu Sep 17 23:39:52 2015] ata2.00: configured for UDMA/133
[Thu Sep 17 23:39:52 2015] PM: resume of devices complete after 5590.160 msecs
[Thu Sep 17 23:39:52 2015] PM: Finishing wakeup.
[Thu Sep 17 23:39:52 2015] Restarting tasks ... done.
[Thu Sep 17 23:40:08 2015] NVRM: API mismatch: the client has the version 346.82, but
NVRM: this kernel module has the version 346.72. Please
NVRM: make sure that this kernel module and all NVIDIA driver
NVRM: components have the same version.
[Thu Sep 17 23:40:08 2015] NVRM: nvidia_frontend_ioctl: minor 255, module->ioctl failed, error -22
[Thu Sep 17 23:46:01 2015] NVRM: API mismatch: the client has the version 346.82, but
NVRM: this kernel module has the version 346.72. Please
NVRM: make sure that this kernel module and all NVIDIA driver
NVRM: components have the same version.... etc
I have now installed kernel module 346.82 and it is now ok.
This has never happened before.