Screen resolution was ok but is now too low

I looked up the question but I couldn't find a solution...

After some updates and a restart the screen resolution is fairly low (800x600). The only options in the preferences are now 800x600 and 640x480.
Before the restart the resolution was ok (I don't know, which resolution it was exactly... My guess is 1280x720 or something like that).
As I looked up my issue, I read something of xorg. So I wanted to adjust my xorg settings in /etc/X11/xorg.conf. But that file does not exist. When I want to run a "X -configure", I get the following:

Fatal server error:
Server is already active for display 0
 If this server is no longer running, remove /tmp/.X0-lock
 and start again.

Please consult the The X.Org Foundation support
  at http://wiki.x.org
 for help.

 ddxSigGiveUp: Closing log

I couldn't find anything on x.org either...

Here is my lspci:

00:00.0 Host bridge: Intel Corporation Core Processor DRAM Controller (rev 02)
00:01.0 PCI bridge: Intel Corporation Core Processor PCI Express x16 Root Port (rev 02)
00:1a.0 USB Controller: Intel Corporation 5 Series/3400 Series Chipset USB2 Enhanced Host Controller (rev 05)
00:1b.0 Audio device: Intel Corporation 5 Series/3400 Series Chipset High Definition Audio (rev 05)
00:1c.0 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 1 (rev 05)
00:1c.2 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 3 (rev 05)
00:1c.3 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 4 (rev 05)
00:1d.0 USB Controller: Intel Corporation 5 Series/3400 Series Chipset USB2 Enhanced Host Controller (rev 05)
00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev a5)
00:1f.0 ISA bridge: Intel Corporation Mobile 5 Series Chipset LPC Interface Controller (rev 05)
00:1f.2 SATA controller: Intel Corporation 5 Series/3400 Series Chipset 4 port SATA AHCI Controller (rev 05)
00:1f.3 SMBus: Intel Corporation 5 Series/3400 Series Chipset SMBus Controller (rev 05)
00:1f.6 Signal processing controller: Intel Corporation 5 Series/3400 Series Chipset Thermal Subsystem (rev 05)
01:00.0 VGA compatible controller: nVidia Corporation Device 0a29 (rev a2)
01:00.1 Audio device: nVidia Corporation High Definition Audio Controller (rev a1)
02:00.0 Network controller: Atheros Communications Inc. AR9285 Wireless Network Adapter (PCI-Express) (rev 01)
06:00.0 Ethernet controller: Marvell Technology Group Ltd. Device 4381 (rev 11)
3f:00.0 Host bridge: Intel Corporation Core Processor QuickPath Architecture Generic Non-core Registers (rev 02)
3f:00.1 Host bridge: Intel Corporation Core Processor QuickPath Architecture System Address Decoder (rev 02)
3f:02.0 Host bridge: Intel Corporation Core Processor QPI Link 0 (rev 02)
3f:02.1 Host bridge: Intel Corporation Core Processor QPI Physical 0 (rev 02)
3f:02.2 Host bridge: Intel Corporation Core Processor Reserved (rev 02)
3f:02.3 Host bridge: Intel Corporation Core Processor Reserved (rev 02)

I hope, you can help me. If there is anything you need in addition, I will try to deliver it, of course!