I have the problem each time I turn off my monitor and then later turn it back
on. The desktop does not come back.

The monitor is a samsung Samsung S34J55W.

# xrandr
Screen 0: minimum 320 x 200, current 3440 x 1440, maximum 16384 x 16384
DisplayPort-0 connected primary 3440x1440+0+0 (normal left inverted right x
axis y axis) 797mm x 333mm
   3440x1440     59.97*+  74.98  
   2560x1440     59.95  
   2560x1080     60.00    59.94  
   1920x1080     60.00    50.00    59.94  
   1680x1050     59.95  
   1600x900      60.00  
   1280x1024     75.02    60.02  
   1440x900      59.89  
   1280x800      59.81  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    70.07    60.00  
   832x624       74.55  
   800x600       72.19    75.00    60.32    56.25  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    72.81    66.67    60.00    59.94  
   720x400       70.08  
DisplayPort-1 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)
DVI-0 disconnected (normal left inverted right x axis y axis)

# inxi -Fx
System:    Host: luca-pc Kernel: 4.19.16-1-MANJARO x86_64 bits: 64 compiler:
gcc v: 8.2.1 Desktop: KDE Plasma 5.14.5 
           Distro: Manjaro Linux 
Machine:   Type: Desktop Mobo: ASRock model: Z370 Extreme4 serial: <filter>
UEFI: American Megatrends v: P1.80 
           date: 03/20/2018 
CPU:       Topology: 6-Core model: Intel Core i7-8700K bits: 64 type: MT MCP
arch: Kaby Lake rev: A L2 cache: 12.0 MiB 
           flags: lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips:
88728 
           Speed: 800 MHz min/max: 800/4700 MHz Core speeds (MHz): 1: 799 2:
800 3: 800 4: 800 5: 801 6: 800 7: 801 8: 800 
           9: 800 10: 800 11: 800 12: 800 
Graphics:  Device-1: Advanced Micro Devices [AMD/ATI] Pitcairn XT [Radeon HD
7870 GHz Edition] vendor: Gigabyte driver: radeon 
           v: kernel bus ID: 01:00.0 
           Display: x11 server: X.Org 1.20.3 driver: ati,radeon unloaded:
modesetting resolution: 3440x1440~60Hz 
           OpenGL: renderer: AMD PITCAIRN (DRM 2.50.0 4.19.16-1-MANJARO LLVM
7.0.1) v: 4.5 Mesa 18.3.2 direct render: Yes 
Audio:     Device-1: Intel 200 Series PCH HD Audio vendor: ASRock driver:
snd_hda_intel v: kernel bus ID: 00:1f.3 
           Device-2: AMD Cape Verde/Pitcairn HDMI Audio [Radeon HD 7700/7800
Series] vendor: Gigabyte driver: snd_hda_intel 
           v: kernel bus ID: 01:00.1 
           Device-3: Logitech Webcam C210 type: USB driver:
snd-usb-audio,uvcvideo bus ID: 1-8:3 
           Sound Server: ALSA v: k4.19.16-1-MANJARO 
Network:   Device-1: Intel Ethernet I219-V vendor: ASRock driver: e1000e v:
3.2.6-k port: f000 bus ID: 00:1f.6 
           IF: enp0s31f6 state: up speed: 100 Mbps duplex: half mac: <filter> 
Drives:    Local Storage: total: 1.14 TiB used: 408.99 GiB (35.1%) 
           ID-1: /dev/nvme0n1 vendor: Samsung model: SSD 960 EVO 250GB size:
232.89 GiB 
           ID-2: /dev/nvme1n1 vendor: Samsung model: SSD 960 EVO 500GB size:
465.76 GiB 
           ID-3: /dev/sda vendor: Samsung model: HD103SJ size: 931.51 GiB 
Partition: ID-1: / size: 193.99 GiB used: 85.41 GiB (44.0%) fs: ext4 dev:
/dev/nvme0n1p2 
           ID-2: swap-1 size: 34.49 GiB used: 0 KiB (0.0%) fs: swap dev:
/dev/nvme0n1p3 
Sensors:   System Temperatures: cpu: 30.0 C mobo: 30.0 C gpu: radeon temp: 37 C 
           Fan Speeds (RPM): fan-1: 580 fan-2: 825 fan-3: 0 fan-4: 562 fan-5:
577 
           Voltages: 12v: N/A 5v: N/A 3.3v: 3.28 vbat: 3.18 
Info:      Processes: 259 Uptime: 26m Memory: 31.35 GiB used: 2.24 GiB (7.1%)
Init: systemd Compilers: gcc: 8.2.1 clang: 7.0.1 
           Shell: fish v: 3.0.0 inxi: 3.0.30